1. Field of the Invention
The present invention relates generally to a heat roller fixing apparatus for thermally fixing a toner image on a sheet and, more particularly, to a heat roller fixing apparatus for preventing an unevenness in temperatures during a standby status.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile, etc., a latent image forming type recording apparatus like an electrophotographic apparatus has been utilized. In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive drum and thereafter developed, thus forming a toner image. Then, after transferring the toner image on the photosensitive drum onto a sheet, the toner image is fixed onto the sheet by heating.
A heat roller fixing device, a flash lamp fixing device and a pressure fixing device may be given as this type of fixing device. Among these fixing devices, the heat roller fixing device has been widely utilized because of its simple construction. This heat roller fixing device effects preheating in a standby status. It is desirable that the thermal fixation be immediately stably done upon an incoming of a print command from this standby status.
The heat roller fixing device, heats up the roller by use of halogen lamps. Then, the fixing is performed with the aid of the heat thereof and a pressure between the former roller and a roller on the opposite side while a sheet is sandwiched in therebetween.
On the other hand, a fixing energy E needed for the fixation is a sum of an energy E1 required mainly for melting a toner and an energy E2 absorbed by the sheet. In the color image forming apparatus, with respect to these energies E1 and E2, when a fixing energy E for monochromatic printing is compared with a fixing energy E for color printing, the energy E2 absorbed by the sheet does not change. Because of factors such as a toner layer thickness, etc., however, the energy E1 needed for melting the toner changes depending on the toner layer thickness.
In a comparison in terms of the toner layer thickness between the monochromatic printing and the color printing, the monochromatic printing has a toner layer thickness for one layer. Contrastingly, the color printing entails a superposition of colors and therefore has two layers in the case of multi colors (seven colors) but four layers in the case of full colors. For this reason, the color printing requires a larger energy needed for melting the toner than in the monochromatic printing.
Therefore, the fixing energy is set as an energy needed for the fixing in the full color printing, and the fixing is, it can be considered, done with this energy in the chromatic printing, too. According to this method, however, the energy necessary for the fixing in the full color printing is twice or above as large as the energy for the monochromatic printing, and therefore, in the case of the monochromatic printing, the electric power is dissipated with a large futility.
Similarly, when an ambient temperature is high, the fixing with a large amount of energy leads to the futility of the electric power dissipated. As a method of preventing this, it is proposed that a plurality of halogen lamps are provided within the heat roller.
Based on this method, the minimum electric power quantity required can be supplied depending on the print types, i.e., the monochromatic printing and the color printing as well as on a magnitude of the ambient temperature. The dissipation quantity of the electric power can be thereby reduced.
There arise, however, the following problems inherent in the prior art.
The heat roller fixing device is required to immediately perform the fixing upon the incoming of the print command in the standby status where the print command does not yet come. For this reason, such a heat roller fixing device effects preheating at a temperature (e.g., 160.degree. C.) lower than a fixing temperature (e.g., 180.degree. C.) in the standby status.
In the standby status where the heat roller temperature is the fixing temperature or under, the toner is fixedly adhered to a cleaner and a sheet separation pawl that are brought into contact with the heat roller. Therefore, when rotating the heat roller, the surface of the heat roller is easy to damage. For this reason, the heat roller is not rotated in this standby status.
Accordingly, the heat roller equipped with the plurality of halogen lamps serving as the above-mentioned heating members performs the preheating by use of some of the halogen lamps. This conduces to such a problem that the surface temperatures of the heat roller are not uniform. Hence, even when heated at the above fixing temperature, there arises a problem in which the unevenness in the fixing appears due to the ununiformity in the surface temperatures when the initial printing is conducted.